The present invention relates to image sensing apparatus which convert an optical image into electrical signals representative of the image, and more particularly to image sensing apparatus which utilize an array of integrated semiconductor photosensors.
Image sensing apparatus which utilize an array of semiconductor photosensors are known in the art, and are used in a variety of equipment, such as television cameras and automatic focussing systems for cameras. One widely used type of image sensing apparatus includes an array of charge couple devices (CCD's) integrated on a single chip of semiconductive material. Such chips provide for sensing of an optical image and for scanning of the image to provide electrical signals representative of the image. However, for applications in automatic focussing systems, the size limitation on the photosensor array are such that CCD array chips made according to present technology are too large. Therefore, to use CCD arrays in such applications, it is necessary to reduce the size of the CCD chips by increasing the packing density in such chips through a reduction in the electrode dimensions of the CCD cells and the spacing between the cells in the array. But, a reduction in the electrode dimensions of the CCD cells reduces the charge storage capacity of the cells, and thereby decreases the dynamic range of the image sensing system. As a consequence of the reduced dynamic range, a CCD array small enough for use in an automatic focussing system is susceptible to the problem of image blooming. Moreover, increasing the packing density in CCD chips manufactured by present technology would also reduce the manufacturing yield of such chips, and therefore would increase the cost of such chips.
Smaller photosensor arrays can be achieved without sacrificing dynamic range or manufacturing yield by replacing the CCD arrays with arrays of photodiodes. However, prior image sensing apparatus using photodiode arrays have problems which will now described with aid of FIG. 1. With reference to FIG. 1, there is shown a schematic circuit diagram of a photodiode image sensing array 100, illustrated as a one-dimensional array. The photodiodes 2 are arranged in the focal plane of an imaging system (not shown in FIG. 1) and serve to detect the light intensity in a respective portion of an image in the focal plane. Each photodiode is connected to a capacitance 3, which stores a voltage related to the intensity of light detected by the photodiode, and a transfer gate (a field-effect transistor) 4 which operatively couples the voltage on the capacitance 3 to a common sense line 6. The transfer gates 4 are sequentially gated by gating signals provided by a shift register 1, and the voltages on the capacitances 3 are sequentially placed on the common sense line 6. The sense line 6 is connected to an analog-to-digital (A/D) converter 5 which converts the analog voltage on the sense line 6 to a digital code representative of the voltage. Thus, the A/D converter 5 provides a sequence of digital codes representative of the intensities of light detected by the photodiodes 2 of the array.
One problem with the photosensor arrangement of FIG. 1 is that as the array became larger, the ratio of the parasitic capacitance C.sub.L of the sense line 6 to the value of the capacitance C associated with each photodiode becomes larger. As a consequence, the signal-to-noise ratio of the voltage signal on the sense line 6 becomes smaller. The main source of noise on the sense line 6 is the switching noise from the gating of the transfer gates 4.
One known solution to the signal to noise problem in the prior art photodiode image sensor arrays is to decrease the capacitance ratio C.sub.L /C by dividing the sense line 6 into several parts. However, a drawback of this solution is that each part of the sense line requires a separate A/D converter, and the use of multiple A/D converters causes the size of the chip to become too large. Another known solution is to the signal-to-noise problem of prior art photodiode image sensor arrays is to provide each photodiode with associated circuitry for providing a digital code representing the photocurrent of the diode. However, owing to the necessarily simple nature of such encoding circuits, the digital codes provided by such circuits generally do not contain sufficient information to accurately represent the photocurrent of the photodiode. Therefore, a need clearly exists for a photodiode array type image sensing apparatus which has a relatively high signal-to-noise ratio and which provides sufficient digital information for accurate represenation of the image for the purposes of comparing images, and yet be of simple construction and easily manufactured.